Getting a slab pour right

Bag counts are the easy part. These are the things that decide whether the slab is still flat in five years.

• Forgetting the base. Concrete poured onto soft ground will crack no matter how many bags you buy. Compacted gravel first.
• Mixing too wet. Extra water makes it easier to place and markedly weaker. Follow the water on the bag.
• Running out mid-pour. A cold joint between yesterday's concrete and today's is a permanent weak line. Have every bag on site before you start.

Rebar and control joints

#3 or #4 rebar on an 18 in grid suits most residential slabs, kept up off the ground on chairs so it sits in the middle of the pour rather than the bottom. Cut control joints at roughly 2.5 times the slab thickness in feet — about every 10 ft at 4 in. Concrete cracks; joints decide where.

Bag yields are not exact

Bag yields describe the finished volume of mixed CONCRETE, not the dry weight of the powder. An 80 lb bag makes about {BAG_YIELD[80]} cu ft, a 60 lb bag {BAG_YIELD[60]}, a 40 lb bag {BAG_YIELD[40]} — so three 40 lb bags do not equal one 80 lb one, and buying the smaller size costs more per cubic foot as well as being more bags to open.

Brands differ by a few percent either way. The 5% margin on every count here absorbs that along with the low spots every real excavation has.

Why one cubic yard is where bags stop making sense

One cubic yard is 27 cubic feet — roughly a 9x9 slab at 4 in thick, or 48 bags of 80 lb. Below that, bags win easily: ready-mix suppliers have a minimum load and a short-load fee, so a small delivery is expensive per yard.

Above it the argument stops being about money. A slab has to be placed and floated as one piece, before the first concrete in starts to set — you have perhaps 90 minutes in mild weather and much less in the sun. Fifty bags cannot be opened, mixed and placed fast enough by two people to keep a wet edge across the whole pour.

Where fresh concrete meets concrete that has already stiffened, the two do not knit. That is a cold joint, and it is a permanent plane of weakness running through the slab — the crack is decided before you have finished pouring. This is the real reason to order a truck, and it is why "we will just do half today" is the most expensive shortcut on this page.

How these numbers are worked out

Volume is length times width times thickness, converted to cubic feet, divided by the bag yield, plus 5%. Nothing is looked up in a table, so any dimension gives a consistent answer.

The figures assume a flat, properly prepared subgrade and the thickness you enter. A real excavation is never perfectly flat, which is why 5-10% more concrete than the arithmetic suggests is normal — the extra goes into the low spots. Anything structural, or carrying a vehicle or a building, should be sized by someone looking at the site rather than by a calculator.